In the case of welding, metal members similar in kind are generally joined with each other. However, if welding can be applied to joining of dissimilar metal members such as a steel-base material (hereinafter referred to merely as a steel member) and an aluminum-base material (generic term used for pure aluminum and aluminum alloy, and hereinafter referred to merely as an aluminum member) with each other (an aggregate of the dissimilar materials joined), this will enable the aggregate of the dissimilar materials joined to be used as a structural member of the automobile, and so forth, thereby significantly contributing to reduction in weight of the automobile, and so forth.
However, in the case of joining the steel member with the aluminum alloy member by welding, a brittle Fe—Al intermetallic compound is prone to be formed at a joint, so that it has been very difficult to obtain a reliable joint high in strength (bonding strength). Accordingly, to effect joining at those aggregates of the dissimilar materials joined (dissimilar metal members joined), joining by use of bolts, rivets, and so forth has been adopted in the past, however, a bonded joint has a problem with its reliability, air-tightness, cost, and so forth.
Meanwhile, it has been intended to enhance strength of the aluminum member as well as the steel member in order to achieve reduction in weight of automobile components such as an automobile body, and so forth, so that there has been a tendency to use a high-strength steel (high tensile steel) member for the steel member, and to use a high-strength A6000 series aluminum alloy member with less alloying elements, and excellent in recyclability among aluminum alloy members.
In consequence, in the case of joining dissimilar materials with each other by welding, there has since occurred a change in joining objects, that is, a change from the conventional joining of dissimilar materials low in strength with each other, such as joining of mild steel with pure aluminum or a A5000 series aluminum alloy, by welding, to joining of dissimilar materials high in strength with each other, such as joining of the high-strength steel with the A6000 series aluminum alloy member by welding. With the joining of those dissimilar materials high in strength with each other, conditions under which the brittle Fe—Al intermetallic compound is generated at a joint will vary on a case-by-case basis. Hence, in order to obtain reliable, and high bonding strength, it becomes necessary to devise new joining conditions in contrast with conditions for the conventional joining of the dissimilar materials low in strength with each other, by welding.
In the case of joining dissimilar materials such as the steel member and the aluminum member with each other, the steel member is high in melting point, and electrical resistance, but low in thermal conductivity, as compared with the aluminum member, so that heat evolution on the steel side of a joint will be greater, thereby causing aluminum lower in melting point to be first fused. Next, the surface of the steel member undergoes fusion, resulting in formation of a brittle intermetallic compound of Fe—Al series on an interface therebetween. As a result, it is not possible to obtain a high bonding strength.
Accordingly, there have since been made many reviews and proposals on a joining method for obtaining the joint of the dissimilar materials such as the steel member and the aluminum member. For example, a method for joining the dissimilar materials together by rolling the same in a vacuum has been proposed (refer to Patent Document 1). There has also been proposed a method for making seam welds by interposing a two-layer cladding material made up of a steel-base material layer and an aluminum alloy layer, prepared beforehand (refer to Patent Document 2). Further, there has been proposed a method for pressure joining at a high temperature (refer to Patent Document 3). Still further, there has been proposed a method for joining by HIP treatment by interposing a Ti alloy placed on respective joint surfaces (refer to Patent Documents 4 and 5). Yet further, there has been proposed a method for friction welding (refer to Patent Document 6). Then, there has been proposed a method whereby resistance welding is carried out by plating the surface of a steel material layer, in contact with aluminum, with an aluminum alloy beforehand, or by interposing the two-layer cladding material made up of the steel-base material layer and the aluminum alloy layer, prepared beforehand (refer to Patent Documents 7, and 8).
Those conventional techniques, however, each have the following problem. For example, the methods for obtaining the joint of the dissimilar materials such as the steel member and the aluminum member, according to Patent Documents 1 to 8, respectively, are in common with each other in that those methods are applicable to joining of members relatively simple in shape, such as flat sheets, or the like, with each other, but are not applicable to joining of members complex in shape with each other because of constraints imposed on the shape thereof. For this reason, those methods each have a narrow range of application, and are inferior in general versatility. Further, those methods each have a problem in that a joint is confined to a relatively small portion of the area of the joint, thereby preventing a continuous joint from being obtained. Still further, because any of those methods will be complicated in process step, it is not possible to ensure stability in quality, thereby causing a problem of an increase in joining cost, and lack of practicality. Yet further, with the existing welding line, it is not possible to carry out any of those methods, and in order to carry out any of those methods, there is the need for adding new facilities to the present facilities, causing another problem of an increase in capital cost.
As one of the background behind the proposals on the various method for joining the steel-base material with the aluminum-base material, described as above, there can be cited a phenomenon that if the steel-base material is joined directly with the aluminum-base material by fusion, a brittle intermetallic compound is generated at the joint, thereby causing the joint prone to cracking. Accordingly, when joining the steel-base material directly with the aluminum-base material, including the case of joining using a welding wire, it becomes extremely important how to prevent as much as possible steel of the steel-base material, and aluminum of the aluminum-base material from being fused to be mixed with each other to thereby secure ductility of fused metal parts, and how to prevent the brittle intermetallic compound from being formed in the vicinity of an interface between the steel-base material, and the aluminum-base material
In contrast to the above, there has been proposed a method for line-joining or face-joining the steel material with the aluminum material by arc welding (refer to Non-patent Documents 1, 2, and 3). Further, there has also been proposed a method for joining the steel material directly with the aluminum material by MIG brazing in order to secure a sound bonded joint (refer to Patent Document 9).
With the method for joining the steel material with the aluminum material by arc welding, as described in Non-patent Documents 1, 2, 3, and so forth, holes are provided on the steel material side of a joint beforehand, and a growth direction of an intermetallic compound acting as a factor for blocking securing of strength is controlled by filling up the holes with the aluminum material, thereby attempting to obtain a high bonding strength. However, with those methods as described in Non-patent Documents 1, 2, 3, respectively, cracking is prone to occur to beads in the case of continuous arc welding, so that a welded joint still has a room for improvement in strength. The same applied to Patent Document 9.
Further, there has been proposed a method for brazing at a low temperature so as to prevent a brittle Fe—Al intermetallic compound from being generated at a joint (refer to Patent Documents 10, 11).
Still further, in the case of melt welding for those joints of the dissimilar materials, whereby joining is carried out at a higher temperature, there has been proposed a method for joining an aluminum alloy member with a steel member having a surface with zinc plating applied thereto by pulse MIG welding with the use of a solid wire made of an aluminum alloy with addition of silicon at least in a range of 3 to 15 wt % as a welding wire (refer to Patent Document 12). With this method, upon fusion of the welding wire, silicon is caused to move toward a base metal to permeate the interface of a fusion pond, thereby rising in temperature, whereupon wettability of fused metals is improved to thereby enhance adhesion properties thereof.
Further, it has been proposed to enhance the strength of a welded joint by improving composition of a flux for use in melt-welding of the joint of the dissimilar materials. For example, there has been proposed a method for arc welding of a mild steel with a pure aluminum member, or an A5000 series aluminum alloy member by use of a wire with a flux incorporated therein, formed by coating the flux containing a fluoride (cesium fluoride, aluminum fluoride, potassium fluoride, and aluminum oxide), serving as a core material, with aluminum, or an aluminum alloy (refer to Patent Document 13).
Still further, there has been proposed a method for joining dissimilar materials together, that is, joining a steel member with an aluminum member by any of various welding processes such as magnetic welding, ultrasonic welding, high-frequency welding, spot welding, and so forth, using a fluoride-based mixed flux containing potassium fluoride, aluminum fluoride, and so forth, together with at least one fluoride selected from the group consisting of cesium fluoride, aluminum fluoride, potassium fluoride, and zinc fluoride), in as-coated state (refer to Patent Document 14). With those methods as described, cleaning on the surface of the steel is urged by the agency of the chemical reaction of the flux as above, and the wettability as well as adhesion properties of a molten metal composed of aluminum will be improved, thereby blocking formation of a brittle and thick intermetallic compound.
Yet further, there has also been proposed a method for spot welding of a mild steel with an A6000 series aluminum alloy member by coating the surface of the aluminum alloy member with a fluoride-based flux having the effect of causing reduction of a sturdy oxide film formed on the surface of the aluminum alloy member to be thereby melt and removed from the surface of the aluminum alloy member (refer to Patent Document 15). Furthermore, those fluoride-based fluxes are also used for joining aluminum alloy members with each other by melt welding and so forth (refer to Patent Documents 16, and 17).    Patent Document 1: JP-A No. 2000-94162    Patent Document 2: JP-A No. 11 (1999)-197846    Patent Document 3: JP-A No. 10 (1998)-185040    Patent Document 4: JP-A No. 6 (1994)-198458    Patent Document 5: JP-A No. 5 (1993)-8056    Patent Document 6: JP-A No. 8 (1996)-142755    Patent Document 7: JP-A No. 6 (1994)-39558    Patent Document 8: JP-A No. 6 (1994)-63762    Patent Document 9: JP-A No. 2003-33865    Patent Document 10: JP-A No. 7 (1995)-148571    Patent Document 11: JP-A No. 10 (1998)-314933    Patent Document 12: JP-A No. 2004-223548    Patent Document 13: JP-A No. 2003-211270    Patent Document 14: JP-A No. 2003-48077    Patent Document 15: JP-A No. 2004-351507    Patent Document 16: JP-A No. 2004-210013    Patent Document 17: JP-A No. 2004-210023    Non-patent Document 1: WELDING JOURNAL, (1963), p. 302    Non-patent Document 2: LIGHT METAL WELDING, Vol. 16 (1987) No. 12, p. 8    Non-patent Document 3: National Conference of Japan Association of Welding, Proceedings No. 75 (2004), pp. 260 to 261